In order to keep a drug concentration constant in a living body for a long period of time, a method of controlling the release by encapsulating the drug in a microcapsule or hydrogel composed of a bioabsorbable polymer which absorbs the drug has been known. Many kinds of natural or synthetic polymers such as collagen, gelatin, polylactic acid, polyglycolic acid and poly-γ-glutamic acid have been reported to be a bioabsorbable polymer which may be used to such an end.
When a sustained-release preparation is embedded into a living body to allow a drug to be released, it is believed to be advantageous also in terms of alleviation of side effects when sustained release of the drug can be realized in a specified direction such as a direction from the embedded site toward the place where the lesion is present, because impairment and damage of peripheral normal tissues resulting from release of the drug toward directions other than the specified direction can be suppressed. However, although a variety of methods have been attempted hitherto in regard to control of the velocity of release, no method has been known allowing control of the direction of release.
Japanese Patent No. 2702729 discloses a sustained-release embedded agent prepared by laminating or adjoining two kinds of matrices, i.e., a matrix obtained by mixing a biodegradable polymer with a biologically active substance, and a matrix comprising a biodegradable polymer alone. The object of this invention is to provide a sustained-release embedding agent enabling control of the velocity of release of a biologically active substance, but control of release directionality is not mentioned.
An object of the present invention is to provide a sustained-release preparation which can control the direction of release of a drug.